The present invention relates to a sensitized sheet for use in a pressure sensitive copy system and particularly the sensitized sheet having a coating comprising an acceptor which is capable of color forming when coming into contact with a colorless chromogenic compound.
Generally, the pressure sensitive copy system utilizes a color forming reaction between an electron donating colorless compound and an electron accepting solid acid.
Various types of the pressure sensitive recording sheets are hitherto known. For example, U.S. Pat. No. 2,730,456 discloses a transfer type of the pressure sensitive recording sheets wherein an upper sheet or overlying sheet is coated with a layer containing microcapsules in which an electron donating colorless compound (hereinafter referred to as "color former") dissolved in an oily solvent is enveloped, and an underlying sheet is sensitized with a coating layer containing an electron accepting solid acid (hereinafter referred to as "acceptor"). The color former transfers into the underlying sheet upon rupture of the micro-capsules and forms a visible colored image on the underlying sheet. If multiple copies are desired, an intermediate sheet carrying the micro-capsules on one side and the acceptor on the other side is inserted between the upper sheet and the underlying sheet.
Also, U.S. Pat. No. 2,730,457 discloses a recording sheet wherein both of the fine capsules and acceptor are coated on the same side, so called "self contained copying sheet".
Furthermore, German Pat. No. 1,275,550 discloses a pressure sensitive recording sheet wherein a record forming components soluble in a liquid solvent is carried on the surface and/or inside of a support and said solvent is present isolated from at least one of said record forming components by pressure-rupturable capsules.
Examples of the color former include Leuco type of chromogenic compounds such as Crystal Violet Lactone, Benzoyl Leucomethylene Blue, Malachite Green Lactone, Rhodamine B Lactone, fluoran derivatives and spiropyranes.
Known acceptors include acid clay, activated clay, attapulgite, kaolin and other inorganic solid acids, but there are disadvantages that a developed color image is faded by the action of moisture and sunlight. A sensitive sheet coated with organic solid acid such as phenolic resins is also used. However such sheet is apt to yellow by the sunlight and the developed color image is decreased in dense or allowed to disappear for a little while by heat or moisture.
As another examples of the organic solid acid, aromatic carboxylic acids (U.S. Pat. Nos. 3,322,557 and 3,488,207) and polyvalent metal salts of aromatic carboxylic acid (DT-OS 2,152,765) are proposed. There are, for example, described benzoic acid, o-nitrobenzoic acid, o-chlorobenzoic acid, 4-methyl-3-nitrobenzoic acid, p-isopropylbenzoic acid, p-tert.-butylbenzoic acid, salicylic acid, 5-tert.-butylsalicylic acid, 3-cyclohexylsalicylic acid, 3-methyl-5-isoamylsalicylic acid, 3,5-dinitrosalicylic acid, 1-naphthoic acid, 1-hydroxy-2-naphthoic acid, 5,5'-methylen-disalicylic acid and the other similar aromatic carboxylic acids, and salts of metals such as magnesium, calcium, zinc, cadmium, aluminum, gallium, tin, lead, chromium, molybdenum, manganese, cobalt and nickel with the above carboxylic acids. Such aromatic carboxylic acids and polyvalent metal salts thereof are superior in stability toward the sunlight to the phenolic resins. Some of the aromatic carboxylic acids and polyvalent metal salts thereof have a sublimating property, and therefore a sheet sensitized by a coating containing such acceptors loses a color-forming ability with the lapse of time. Since the said acceptors are relatively soluble in water and thus diffuse within the sheet by the action of high moisture or water, the color-forming ability is lost on the surface of the sheet. Such disadvantages show a increasing tendency as the molecular weight of aromatic carboxylic acid is lowered. The reason why salicylic acid was formerly proposed but not used in practice is that it has disadvantages as mentioned above as well as a weak point of lower color density. Some of aromatic carboxylic acids and polyvalent metal salts thereof show improved resistance toward heat and moisture as the molecular weight increases and can maintain relatively stable color-forming properties at the normal temperture and humidity. The pressure sensitive recording papers, however, leave room for further improvements of the properties to be required in practice. It is unavoidable that the recording papers are stored and handled under the condition of high humidity or water attaches directly to the sheet. For example, it often occurs that the rainwater or water spilt from a glass attaches to the sheet. In some cases, when a letter or figure is printed by offset printing on the upper or underlying sheet of pressure sensitive recording papers, fountain solution on the blanket transfers onto an acceptor coated surface of the sheet. Under such circumstance, if an organic acid substance has insufficient resistance toward high moisture or water, a sensitized sheet is markedly reduced in the color-forming ability or in case the sensitized sheet is in touch with a sheet carrying the encapsulated color former the organic acid substance comes into contact with the color former by means of the moisture or water as a carrier and thus there arises undesirable color which is so called "smudge".
Accordingly, a coating layer containing the acceptor must have a high humidity or water resistance sufficient to maintain stabilities of the color-forming ability and developed color image and to inhibit the "smudge".
On the other hand, a coating composition containing aromatic carboxylic acids and polyvalent metal salts thereof, because of being unstable toward heat in general, needs a careful watch and handling during storage or coating procedure. Paticularly, in the coating procedure the coating composition is subject to mechanical shear and accompanied by a rising temperature. Therefore, when a mechanical and thermal stability is insufficient, the coating composition cannot form a uniform layer on the surface of a base sheet and in the worst case it is unavoidable to discontinue the coating procedure. If the coating procedure is effected by a coating apparatus mounted on a paper machine, it is the most simplified process and therefore advantageous economically. In this case since the coating composition is coated on the base sheet preheated by a dryer, the mechanical and thermal stability requirements become more severe.